Many related art engine systems utilize skip-firing modes or fueling modes. These skip-firing modes include various skip-firing patterns and various fueling strategies, but they do not provide a solution for increasing the diesel substitution factor (DSF), that is, decreasing the amount of diesel fuel that is consumed and replacing it with another fuel to provide the desired amount of total energy to fulfill the demanded engine load. Because diesel fuel is readily available, and its ignition properties are well known, diesel fuel is often used as a pilot fuel for triggering combustion of other fuels that are less readily ignited, such as natural gas or other gaseous fuels. However, other substances, such as dimethyl ether or kerosene, could be substituted as the pilot fuel. Accordingly, references to “diesel” and “DSF” will be understood to include fuels that can be employed as pilot fuels to trigger the combustion of a different fuel, which is employed as the main fuel. On average, the main fuel constitutes the majority of the fuel that is consumed by the engine.
U.S. Pat. No. 5,553,575 relates to a “gas-fueled unthrottled internal combustion engine” having an excess air ratio (lambda) that is optimized by selecting automatically and continuously the optimum fraction of combustion chambers firing (OFF) as a function of engine operating parameters. Further lambda adjustment is performed by suitable control of exhaust gas recirculation (EGR), ignition timing, and/or turbo air bypass (TAB). More specifically, the '575 patent discloses a dual-fuel system which can be fueled with port-injected natural gas and an ignition assist system that can consist of a spark plug or a fuel injector for introducing pilot quantities of diesel fuel directly into the combustion chamber. Port-injected natural gas is injected into the intake port, upstream of the engine intake valve so that the natural gas mixes with the intake air during the engine's intake and compression stroke. If the mixture of natural gas and air detonates prematurely during the compression stroke, this premature detonation is commonly referred to as “engine knock” and this can result in significant damage to the engine. To reduce the risk of engine knock, engines normally reduce the compression ratio and/or reduce the amount of natural gas and increase the amount of diesel fuel that is consumed by the engine. Compared to a conventional diesel engine, in which the fuel is injected directly into the combustion chamber late in the compression stroke, forming a stratified charge, fuel that is port-injected has more time to mix with the air, forming a more homogeneous mixture. The '575 patent also discloses that, for port-injected natural gas, it is important to maintain lambda within a narrow range for the efficient combustion of the homogeneous mixture and to avoid misfiring and excessive production of NOx. Accordingly, the '575 patent is directed to a method for controlling lambda; it is not directed towards a method of reducing the amount of diesel fuel consumed.
Like the '575 patent, U.S. Pat. No. 5,477,830 also relates to an internal combustion engine with natural gas that is injected into the intake air system, upstream of the combustion chamber intake valves. However, the '830 patent is specifically directed to an electronic fuel injection system for the precise distribution of natural gas into each cylinder for engines that use a shared intake port for a pair of cylinders. Duration and timing of the fuel injection pulse and other injection strategies, such as a skip-fire operation, are controlled and enabled. However, like the '575 patent, because the '830 patent is directed to a dual fuel engine that teaches fumigating the natural gas to form a homogeneous mixture and controlling the air-fuel mixture (lambda); its objective is not increasing the DSF.
With an engine that injects a main fuel into the intake air system, and injects a second fuel, such as diesel fuel directly into the combustion chamber, there are times when the DSF is decreased (not increased). For example, when the amount of port injected fuel is limited to prevent, or at least reduce, engine knock, the amount of injected diesel fuel is increased to satisfy the total amount of energy needed for the commanded engine load and speed condition. With engines such as those disclosed in the '575 patent and the '830 patent it is common under some normal operating conditions for the fuel delivered to the engine to comprise between 50% and 100% diesel fuel.
Skip-fire techniques are utilized by some conventional gasoline or diesel mono-fueled engines, but for engines that are fueled with only one fuel, DSF is not applicable. Rather, there are various other reasons for using skip-fire techniques combined with different fuel injection strategies, for example, to reduce smoke emissions, to increase boost pressure, and to adjust air to fuel ratio.
U.S. Pat. No. 5,826,563 relates to a high horsepower locomotive diesel engine that is operated in a skip-firing mode, wherein the engine includes a plurality of individually controllable, fuel-injected cylinders. The system senses that the engine is operating in a low horsepower mode and has a low fuel demand. The pattern selected for firing the cylinders is arranged such that all cylinders of the engine are fired within a preselected number of crankshaft rotations. The system also senses the engine air-fuel ratio and adjusts the pattern of cylinders being fired so as to maintain exhaust emissions below a preselected level. Additionally, the pattern of fired cylinders can be adjusted to maintain engine operating temperature and as a function of engine speed. Accordingly, the '563 patent relates to skip-fire for the purpose of adjusting the air to fuel ratio and adjusting the total fuel limit value for reducing smoke emissions in locomotive diesel systems.
U.S. Pat. No. 6,405,705 and continuation-in-part U.S. Pat. No. 6,823,835 both relate to a diesel engine having a plurality of individually controllable fuel-injected cylinders that is operated in a skip-firing mode to reduce smoke emissions during low power operation. The system senses certain identified engine operating parameters and, when these parameters exceed predetermined thresholds for a predetermined time, skip-firing is implemented. Upon implementation of skip-firing, the engine timing angle is reset by a fixed angle and a multiplication factor is included in the speed loop integrator to ensure that the appropriate fuel volume value is injected into each cylinder immediately upon initiation of skip-firing. The '705 patent relates to skip-fire in conjunction with adjusting the air to fuel ratio and adjusting the total fuel limit value for reducing smoke emissions in locomotive engine systems, and the '835 patent relates to adding fuel from skipped cylinders into fueled cylinders for adjusting air to fuel ratio in order to maintain performance parameters.
U.S. Pat. No. 6,408,625 relates to an electric power generation system which includes a back-up electric power generator driven by a four-cycle internal combustion engine. The engine includes a compressor along an intake pathway to deliver pressurized air to the cylinders and a turbine along an exhaust pathway to power the compressor when driven by exhaust discharged from the cylinders. The engine is prepared to accept a generator load by increasing boost pressure provided by the compressor. This increase in boost pressure is accomplished by skip-firing the cylinders in a selected pattern, thereby retarding ignition timing for the cylinders, or by using a combination of these techniques. Accordingly, the '625 patent relates to a skip-firing technique for increasing boost pressure.
U.S. Pat. No. 8,136,497 involves a method for improving starting of an engine that can be repeatedly stopped and started to improve fuel economy. In one embodiment, the method involves using skip-fire when the engine is idling to reduce fuel consumption and prevent the engine speed from overshooting the desired idle speed. Another embodiment is disclosed whereby skip-fire is employed for torque control.
U.S. Patent Application Publication No. 2011/0253113 relates to an engine that is configured with an exhaust gas recirculation (EGR) system, The EGR system comprises exhaust manifolds from one or more cylinders being connected to an intake system, such cylinders being referred to as donor cylinders. These donor cylinders are the cylinders from which exhaust gas is recirculated to the intake. For an engine which uses skip-fire, the '113 publication relates to various methods and systems for operating an internal combustion engine that has one or more donor cylinders and one or more non-donor cylinders. Accordingly, depending upon the engine operating conditions, the '113 publication is directed to methods for choosing whether to skip either donor cylinders or non-donor cylinders when skip-fire is commanded. For example, during an EGR cooler heating mode, the system operates at least one of the donor cylinders at a cylinder load that is sufficient to increase an exhaust temperature for regenerating an EGR cooler and operates at least one of the non-donor cylinders in a low-fuel or no-fuel mode.
U.S. Pat. No. 8,131,447 relates to a variety of methods and arrangements for improving fuel efficiency of internal combustion engines, including selectively skipping combustion events so that other working cycles can operate at a better thermodynamic efficiency. A controller is used to dynamically determine the chamber firings required to provide the engine torque based on the engine's current operational state and conditions. The chamber firings can be sequenced in real time or in near real time in a manner that helps reduce undesirable vibrations of the engine.
While these background examples may relate to skip-fire techniques in association with a variety of technical problems, they fail to disclose an engine that injects two different fuels directly into the combustion chamber, or methods for increasing the amount of one fuel that is substituted for the other fuel. More specifically, when diesel fuel is employed as a pilot fuel, none of these background examples discloses increasing DSF and reducing overall diesel pilot fuel consumption in direct-injection compression-ignition engine systems.